ON THE PENETRATION OF THE 660 KM PHASE-CHANGE BY MANTLE DOWNFLOWS

We present a simple analytic model of the interaction of cold convecti ve downwelling currents with an endothermic phase change. The model de scribes the ponding and lateral spreading of downflows along the phase transition interface. A simple comparison of the vertical forces on t he ponding material provides a necessary condition for a downflow to p enetrate the phase boundary. This condition is fundamentally dependent on the geometry of the downflow. For planar downwellings, the model p redicts a minimum ponding time before the structure can penetrate the phase boundary. For columnar (axisymmetric) downflows, there is no min imum time of spreading required before penetration can proceed. The mo del thus provides an explanation for the observation that in numerical models of three-dimensional convection with an endothermic phase chan ge, cylindrical downflows penetrate the phase interface while planar o nes do not. Since descending slabs in the Earth's mantle display a wid e spectrum of geometries between planar and cylindrical (given various trench curvatures, as well as intersections of two or more subduction zones), this phenomenon may explain, in part, why some slabs appear t o extend into the lower mantle while others are deflected at the 660 k m discontinuity.